This invention relates to inspection systems and, more specifically, to optical spot scanning systems for use in three-dimensional object inspection.
Many three-dimensional inspection systems utilize well known optical triangulation techniques as a basis of operation. In systems of this type, a projector projects a structured light pattern onto the object to be inspected. A television camera or other suitable light sensing device is simultaneously directed at the object and observes the contour or image formed by the intersection or confrontation of the projected light pattern and the object surface. The sensing device then converts the observed image into corresponding electrical signals. By appropriately selecting the angular relationship of the light sensing device and the projector, the three-dimensional relationship of the points on the object surface may be determined from the generated electrical signals.
Once the inspection system is calibrated, the three-dimensional characteristics of the entire surface of any arbitrary object may be determined. In particular, this can be accomplished by moving the object to successive positions in front of the sensor-projector combination, by moving the sensor-projector combination relative to the object or by scanning the projected light pattern over the object in sychronism with a scanning sensor. In each case, at each location of the object relative to the sensor-projector combination, three-dimensional data is collected for that location. This continues until sufficient data is obtained to entirely reproduce the object surface.
In some types of inspection systems, a plane of light is projected onto the object surface and the three-dimensional characteristics of the object are determined along the profile formed by the intersection of the light plane and the object surface. By creating relative motion between the light plane and the object, intersection profiles are generated over the entire surface and result in the desired three-dimensional data. In this system, a planar light pattern is used because it enables the generation of three-dimensional data over an entire profile which may contain a large number of object surface points.
The aforesaid planar technique, however, has certain drawbacks when the surface to be reproduced is specularly reflective. With this type of surface, repeated or multiple reflections from the object surface may produce false profile images. This results in incorrect data generation and, as a consequence, an erroneous three-dimensional reproduction.
One way of avoiding this multiple reflection effect is to project the light as a spot or point instead of a plane. In this case, the intersection or confrontation of the light beam and the object surface results in the illumination of a primary object surface location and possibly one or more reflections. A linear array of light sensing elements can then be used to sense the primary image of the illuminated surface location, since only one data point is usually illuminated in a particular plane passing through the light beam. However, for the linear array to properly sense the image and exclude the extraneous reflections, the array axis (defined as the direction along the array length) and the optical axis of the sensor imaging system must be maintained coplaner with the projection axis of the projector.
Since multiple surface reflections generally result in illuminating surface locations which are not in the plane defined by the line array, the imaging system optical axis and the projection axis, they are not simultaneously imaged on the line array with the object surface points under investigation. The use of a projected point or spot system thus effectively eliminates multiple reflection problems associated with specular surfaces.
As can be appreciated, however, with a single spot projection system data is collected for only one object surface point at a time. Furthermore, to complete the three-dimensional reproduction procedure, relative motion along two axes must be carried out. This results in increased equipment complexity and increased time to generate the reproduction.
One way of overcoming the aforesaid drawbacks of single spot projection systems is to place an optical scanning device, such as a for example, a scanning mirror, in the path of the projected spot and image light path. This mirror scans the spot over the object surface to, in effect, form a plane of light. However, at each position in the scan cycle, the scanning spot illuminates a single object surface point so that the system behaves precisely like the projected spot system for each object surface location. As a result, multiple surface reflection effects do not degrade system performance.
In order for the object points to be properly sensed in a scanned spot projection system, the above-discussed coplanar relationship of the sensor array axis, the imaging system optical axis and the projection axis must be maintained. To realize this the imaging optical axis must also be scanned in a manner which tracks the projection axis.
One technique which could be employed to accomplish this tracking is to use two separately controlled scanning mirrors, one for the projector and one for the sensor-imaging system combination. The use of multiple scanning mirrors, however, requires additional controls for synchronizing the mirror scan angles. This, in turn, increases system complexity, as well as system cost and size and requires extremely critical component alignment. In addition, these alignments must not drift with time or temperature and must be maintained over the life of the system.
It is therefore an object of the present invention to provide a spot scanning system for use in three-dimensional reproduction and inspection which does not suffer from the above disadvantages.
It is a further object of the present invention to provide a system of the aforesaid type in which a coplanar relationship between the projection axis and the imaging system optical axis is maintained for all scan positions in a cost effective manner which minimizes the components to be synchronized and aligned.